1. Field of the Invention
The present invention relates to a digital apparatus for controlling an operation of an electric motor which controls a speed of the electric motor by utilizing digital processings.
2. Description of the Background Art
A use of a micro-computer in the field of controlling has become increasingly popular in conjunction with the development of the micro-computer, because of a capability of a micro-computer to execute a highly complicated controlling, to such an extent by now that it is almost an indispensable aspect of any controlling.
As an example of such a use of a micro-computer in the field of controlling, a conventional digital speed control for an electric motor will now be described by using a case of an electric motor in an elevator system shown in FIG. 1.
An elevator system of FIG. 1 includes a three phase alternating current power supply 41, a converter 42, a direct current smoothing capacitor 43, an inverter 44, a current detector 45, a coil 46, and an inductive motor 47, such that alternating currents supplied by the three phase alternating current power supply 41 are converted into direct currents by the converter 42 first and then converted into alternating currents of variable voltage and variable frequency by the inverter 44 and finally supplied to the inductive motor 47 as a driving power.
The system further includes a rotation angle detector 48 connected to a motor axis of the inductive motor 47 which has a configuration of either a pulse generator or a resolver and which generates a rotation angle detection signal indicating the detected rotation angle, a sheave 49, a counterweight 50 suspended from one end of a rope wound around the sheave 49, and an elevator car 51 suspended from another end of the rope wound around the sheave 49, such that the elevator car 51 is operated by rotating the sheave 49 with the inductive motor 47. The elevator car 51 is equipped with a weight detector 51a for detecting a weight exerted on a floor of the elevator car 51 and generating a weight detection signal indicating the detected weight.
In this system, the controlling of the speed of the elevator car 51 is achieved by appropriately changing a base driving signals to be given to the inverter 44 such that the currents given to the inductive motor 47 can be appropriately adjusted. A system for this controlling will now be described in detail.
Namely, as shown in FIG. 1, a speed detection signal is obtained from the rotation angle detection signal generated by the rotation angle detector 48 at a rotation angle to speed transformation unit 58, and a difference between the obtained speed detection signal and a speed command signal generated by a speed command generation unit 60 is fed to a speed control unit 59. Then, an output of the speed control unit 59 and the weight detection signal generated by the weight detector 51a are fed to a summing unit 57 which outputs a torque command signal, and this torque command signal and the rotation angle detection signal generated by the rotation angle detector 48 are fed to a vector control processing unit 56 which outputs a current command signal for the inductive motor 47. Then, a difference between this current command signal and a current detection signal generated by the current detector 45 is fed to a current control unit 55 which outputs a voltage command signal. This voltage command signal and an output of a carrier triangular wave generator 54 are fed to a comparator 53 which outputs a base signal, and this base signal is fed to a base driver unit 52 which outputs the base driving signal to the inverter 44 such that the inverter 44 adjusts the voltage and frequency of the alternating currents supplied to the inductive motor 47 according to the base driving signal.
In such a conventional speed control system, a part enclosed by a dashed line in FIG. 1 comprises a digital circuit A which is constructed by a micro-processor and IC elements, and both of the running sequence and the protecting sequence are digitally processed by this digital circuit A. Thus, the current command signal outputted from the vector control processing unit 56 is a digital signal which is subsequently converted into an analog signal by using an A/D converter, such that the current control unit 55, the carrier triangular wave generator 54, the comparator 53, and the base driver unit 52 are operated by analog signals.
However, such a conventional speed control system using a current control by analog signals has been associated with problems that an offset and a circuit constant of an operational amplifier element are affected by an environmental temperature and humidity conditions, and that the analog signals are easily affected by external noises. Moreover, a voltage waveform can be distorted when a loop gain is carelessly increased in order to improve a tracking characteristic of the current control, so that there is a limit to the improvement of the tracking characteristic of the current control.
On the other hand, a recent advance of a digital signal processor (DSP) and an application specific integrated circuit (ASIC) which are capable of high speed digital signal processings enabled a realization of the current control with a digital circuit, such that the aforementioned problems associated with the current control with analog signals can be avoided. However, in such a case of fully digital speed control, a processing speed for the current control and speed control must be faster than that of a micro-processor in order to maintain a consistent operation. This implies that the current control and speed control have to be executed according to a high speed clock which is not synchronous with the micro-processor, and there is a need for an interface between the micro-processor and the current control and speed control. This requirement can complicate a configuration of the system considerably, especially when an ample error prevention is to be provided, which in turn causes problems of a severe restriction on data transfer and an excessive demand on the micro-processor.